Childhood obstructive sleep apnea syndrome (OSAS) appears to be due to a combination of structural and neuromotor factors. It is thought that those children who develop OSAS due to adenotonsillar hypertrophy have additional underlying abnormalities. Magnetic resonance imaging (MRI) studies have shown that adults with OSAS have narrow upper airways compared to controls. It is therefore possible that those children with adenotonsillar hypertrophy and OSAS have narrower upper airways than children with large tonsils but no OSAS. MRI's have also been useful in adults to demonstrate the degree of adipose tissue encroachment on the upper airway. In children, MRIs can be used to measure the size of the upper airway, and establish whether upper airway narrowing, if present, is due to skeletal structures, adipose tissue or lymphoid tissue (tonsils and adinodis). We therefore performed MRIs of the upper airway in 4 children with OSAS (3) of whom received CPAP therapy) and 2 controls. Two studies were technically inadequate due to patient motion (sedation cannot be used in children with OSAS). In the remaining subjects, children with OSAS had smaller upper airways than controls. These very preliminary data suggest that children with OSAS have small pharyngeal airways, and suggest that the patients may be at risk of recurrence of OSAS in later life, should they develop additional risk factors.